1. Field of the Invention
This invention relates to a variable-wavelength light source unit used for evaluating and manufacturing an optical communication system and an optical device.
2. Description of the Related Art
A variable-wavelength light source unit in the related art will be discussed with reference to FIG. 2.
In FIG. 2, the variable-wavelength light source unit in the related art is surrounded by the dotted line.
The variable-wavelength light source unit includes a variable-wavelength light source section 11, a photocoupler 13, an optical power monitor PD (photodiode) 17, and a drive current control circuit 14.
In the configuration in FIG. 2, output of the variable-wavelength light source section 11 is emitted through the photocoupler 13 from an output section of the light source unit.
A part of output light split by the photocoupler 13 is fed into the optical power monitor PD (photodiode) 17.
Output of the power monitor PD 17 is fed into the drive current control circuit 14, which then controls the drive current of the variable-wavelength light source section 11 in response to the output of the power monitor PD 17, thereby controlling the light output from the output section of the light source unit constant.
The variable-wavelength light source section 11 has a configuration as shown in FIG. 3.
In FIG. 3, numeral 31 denotes an LD (laser diode). Light produced by making an electric current flow into the laser diode 31 is made incident on a diffraction grating 33 through a lens 32 at a predetermined incidence angel.
Specific-wavelength light of the light incident on the diffraction grating 33 is reflected on a reflector (mirror) 34 and is output again through the diffraction grating 33, the lens 32, the laser diode 31, and a lens 36.
In the configuration in FIG. 3, the output wavelength is selected as the angle of the mirror 34 is adjusted by a motor 35.
However, the light output characteristic of the variable-wavelength light source section 11 in FIG. 3 has wavelength dependency in which output power Pout of the output light changes depending on the wavelength of the output light as indicated by the dotted line in (2-a) of FIG. 2 (if the drive current of the laser diode 31 is made constant).
It is desirable that the output characteristic of the variable-wavelength light source should be a flat characteristic independent of the wavelength, as indicated by the solid line.
It is considered that the cause may be the effect of change in the control characteristic of the control system caused by fluctuation of ambient temperature, refractive index change caused by local temperature change of the laser diode caused by the electric current flowing into the laser diode, etc.
Hitherto, to provide the flat characteristic as indicated by the solid line in (2-a) of FIG. 2 without wavelength dependency that the output light power of the laser diode changes depending on the wavelength as indicated by the dotted line in (2-a) of FIG. 2, a part of output light has been fed back for controlling the drive current of the laser diode.
However, in the technique in FIG. 2, to correct the wavelength and output level, the drive current of the laser diode is controlled, resulting in a factor impairing the stability of the output light wavelength.
Particularly, to correct the wavelength dependency of the light output level, the drive current value of the laser diode is changed and thus providing a wide range of wavelengths is also inhibited.